The development of portable electronic devices has led to a corresponding increase in the demand for secondary batteries having both a lighter weight and a higher capacity. To satisfy these demands, the most promising approach is a lithium-sulfur battery with a positive electrode including a sulfur-based compound.
Lithium-sulfur batteries use sulfur-based compounds with sulfur-sulfur bonds as a positive active material, and a lithium metal or a carbon-based compound as a negative active material. The carbon-based compound is one that can reversibly intercalate or deintercalate metal ions, such as lithium ions. Upon discharging (i.e., electrochemical reduction), the sulfur-sulfur bonds are cleaved, resulting in a decrease in the oxidation number of sulfur (S). Upon recharging (i.e., electrochemical oxidation), the sulfur-sulfur bonds are re-formed, resulting in an increase in the oxidation number of the S. The electrical energy is stored in the battery as chemical energy during charging, and is converted back to electrical energy during discharging.
The light weight and good energy density of lithium metal has brought about its wide use as a negative active material for lithium sulfur batteries. However, the good reactivity of lithium metal may cause deterioration of cycle life characteristics. Studies regarding a prevention layer have been undertaken in order to address such a shortcoming.
One of the prevention layers evaluated is LIPON (lithium phosphorous oxy-nitride), a lithium ion conductor. The LIPON is formed by sputtering a target material such as Li3PO4 under a nitrogen gas atmosphere. This approach has shortcomings in that nitrogen gas and the Li3PO4 target material react with lithium metal to form a poorly adhering black porous lithium composite compound byproduct on the surface of the lithium metal.
To prevent production of the byproduct, a pre-treatment layer is disclosed in Published U.S. patent application Ser. No. 2002/0012846 A1 (USA, Moltech). The pre-treatment layer includes materials such as Li2CO3, derived from a reaction between gaseous material such as plasma CO2 and a surface of the lithium metal, or metals capable of alloying with lithium, such as copper.
However, the Li2CO3 pre-treatment layer has insufficient lithium ionic conductivity (about 1×10−12 S/cm or less at room temperature), and it causes structural instability due to volume increases. In addition, this process requires additional equipment such as a plasma device, so the production cost is high, and it requires different conditions from the prevention layer to complete the process.